1. Field of the Invention
In modern internal combustion engines, which must meet ever more stringent exhaust gas standards for emissions in both the United States and Europe, in motor vehicles with Otto engines, electrically operated secondary air pumps that can be switched on selectively are used. The secondary air pump is used for exhaust gas posttreatment and shortly after engine starting is activated for a period of between 30 and 60 seconds. During the starting phase, by the admixture of air with the exhaust gas, a thermal postcombustion is brought about, with the purpose of heating up the catalytic converter in the exhaust system faster and increasing its conversion rates.
2. Description of the Prior Art
From the Bosch publication xe2x80x9cOtto-Motoren-Management, Grundlagen und Komponentenxe2x80x9d [Fundamentals and Components of Otto Engine Management], Gelbe Reihe [Yellow Series], 2001 Edition, Technische Unterrichtung [Technical Instruction], page 82, it is known by thermal postcombustion to subject the uncombusted ingredients of the fuel-air mixture that are still present to a postcombustion. If the mixture composition is lean, the requisite oxygen is still contained in the exhaust gas in the form of residual oxygen, since it was not converted during the combustion. However, in a rich mixture, of the kind often required for cold starting internal combustion engines, air (secondary air) introduced additionally into the exhaust conduit can speed up the heating of the catalytic converter. The resultant exothermic reaction on the one hand reduces the uncombusted hydrocarbons and the carbon monoxide, but on the other, the postcombustion also heats up the exhaust gas catalytic converter and brings it to its operating temperature, which enables optimal conversion, more quickly. The optimal conversion rate can thus be reached faster in the warmup phase of the engine, and thus the operating readiness of the catalytic converter is achieved quite fast.
However, it has been found that in intake engines, a considerable portion of uncombusted hydrocarbons (HCs) in the exhaust gas are the product of the preceding engine operation of the engine and are deposited as a film on the wall of the intake tube, or are generated by outgassing from the valves, in particular the valve plates, or involve HC-laden residual gas located upstream of the catalytic converter. These hydrocarbon molecules stay upstream of the exhaust system so long that the next time the engine is started, they are forced in the direction of the catalytic converter. In cold starting, several strokes of the cylinders elapse before the engine starts, and thus this HC-laden residual exhaust gas gets into the cold catalytic converter. There, the HC-laden residual exhaust gas is stored temporarily, and some of it even directly reaches the environment, since the catalytic converter is not yet activated.
With an ensuing temperature increase in the catalytic converter to the dew point, even the HC-laden residual exhaust gas stored temporarily in the catalytic converter reaches the environment uncombusted. Activating the secondary air pump during the starting phase of the engine has no influence on these events described above.
In operation of an internal combustion engine by the method proposed according to the invention, a further reduction in uncombusted hydrocarbons in the exhaust gas can be attained, because the HC residues in the exhaust gas are combusted in the still-hot catalytic converter after an engine operation by supplied secondary air. The residual heat of the catalytic converter can thus be exploited for postcombustion of uncombusted hydrocarbons in the exhaust gas.
An air flow is generated, which transports the HC molecules, adhering to the intake tube, valve plates, and cylinder walls, upstream of the throttle valve through the engine up to the hot catalytic converter. To generate this air flow, both the secondary air pump and a 2/2-way valve can be used. Downstream of the throttle valve, the air flow is introduced into the intake tube in the direction of the inlet valves; the throttle valve is in its closing position, to prevent the secondary air flow generated from escaping in the direction of the air filter. In the engine, in the best case all the inlet and outlet valves of the cylinders are open. In conventional valve control systems in internal combustion engines, an open position of inlet/outlet valves can be achieved at at least one of the cylinders, so that the air flows in the direction of the catalytic converter and becomes enriched with uncombusted HCs.
Besides by the use of a secondary air pump in an internal combustion engine, an air flow can also be generated by providing that the engine is merely turned over by the starter/starter-generator, creating an air flow that flows through the cylinders. For trucks, an air flow in the direction of the catalytic converter can be generated by tapping the compressed air reserve of the brake system.